Differences between Ethereum and OP Stack Chains

OP Stack chains are designed to be EVM equivalent (opens in a new tab) and introduces as few changes as possible to the Ethereum protocol. However, there are some minor differences between the behavior of Ethereum and OP Stack chains that developers should be aware of.

Bridging

Bridging - Deposit Transactions

Deposit transactions don't exist on L1's, and are how transactions on an L2 can be initiated from the L1. Importantly, this is how bridge applications can get L1 ETH or tokens into an L2 OP Stack chain. You can read more on deposit transactions here.

Bridging - Withdrawal Transactions and Fault Proofs

Withdrawal transactions are how the state of the L2 rollup can be proven to the L1. Often this involves users withdrawing tokens or ETH to the L1. Fault proofs are the mechanism by which withdrawal transactions are currently proven to the L1. You can read more about fault proofs here.

Opcodes

Opcode	Solidity Equivalent	Behavior
`COINBASE`	`block.coinbase`	Returns the address of the current Sequencer's fee wallet. Effectively the same as Ethereum with the caveat that the value typically does not change from block to block.
`PREVRANDAO`	`block.prevrandao`	Set pseudorandomly for each block by the Sequencer as opposed to the stronger guarantees provided by RANDAO (opens in a new tab) on Ethereum.
`ORIGIN`	`tx.origin`	If the transaction is an L1 ⇒ L2 transaction triggered by a smart contract on L1, then `tx.origin` is set to the aliased address of the address that triggered the L1 ⇒ L2 transaction. Otherwise, this opcode behaves normally.
`CALLER`	`msg.sender`	If the transaction is an L1 ⇒ L2 transaction triggered by a smart contract on L1, and this is the first call frame (rather than an internal transaction from one contract to another), the same address aliasing behavior applies.

Address aliasing

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Address aliasing is an important security feature that impacts the behavior of transactions sent from L1 to L2 by smart contracts. Make sure to read this section carefully if you are working with cross-chain transactions. Note that the CrossChainMessenger contracts will handle address aliasing internally on your behalf.

When transactions are sent from L1 to L2 by an Externally Owned Account, the address of the sender of the transaction on L2 will be set to the address of the sender of the transaction on L1. However, the address of the sender of a transaction on L2 will be different if the transaction was triggered by a smart contract on L1.

Because of the behavior of the CREATE opcode, it is possible to create a contract on both L1 and on L2 that share the same address but have different bytecode. Even though these contracts share the same address, they are fundamentally two different smart contracts and cannot be treated as the same contract. As a result, the sender of a transaction sent from L1 to L2 by a smart contract cannot be the address of the smart contract on L1 or the smart contract on L1 could act as if it were the smart contract on L2 (because the two contracts share the same address).

To prevent this sort of impersonation, the sender of a transaction is slightly modified when a transaction is sent from L1 to L2 by a smart contract. Instead of appearing to be sent from the actual L1 contract address, the L2 transaction appears to be sent from an "aliased" version of the L1 contract address. This aliased address is a constant offset from the actual L1 contract address such that the aliased address will never conflict with any other address on L2 and the original L1 address can easily be recovered from the aliased address.

This change in sender address is only applied to L2 transactions sent by L1 smart contracts. In all other cases, the transaction sender address is set according to the same rules used by Ethereum.

Transaction Source	Sender Address
L2 user (Externally Owned Account)	The user's address (same as in Ethereum)
L1 user (Externally Owned Account)	The user's address (same as in Ethereum)
L1 contract (using `OptimismPortal.depositTransaction`)	`L1_contract_address + 0x1111000000000000000000000000000000001111`

Transactions

Transaction fees

Transactions on OP Stack chains must pay for an L1 data fee on top of the standard execution gas fee you would expect on Ethereum. Refer to the guide on OP Stack Transaction Fees for more information.

You can use the JS library viem (opens in a new tab) to estimate the entire transaction gas costs, including the L1 Data Fee.

EIP-1559 parameters

The base fee on OP Stack is, like Ethereum, computed via the EIP-1559 (opens in a new tab) mechanism. The EIP-1559 parameters used by OP Stack differ per chain.

Mempool rules

Unlike Ethereum, OP Stack chains do not have a public mempool. The OP Stack mempool is currently only visible to the Sequencer. The Sequencer executes transactions from the mempool in priority fee order (highest fee first).

Chain Finality

Unlike L1s such as Ethereum, OP Stack chains have Unsafe, Safe, and Finalized Heads which indicate the state of finality for a given L2 block. Fault proofs do not impact the finalization of the L2 rollup, only the finalization of withdrawal transactions to the L1. You can read more about these in the docs glossary.

What's Next

There are various useful tools linked above. Here are a few more tools and links you may want to check out:

OP-viem (opens in a new tab): JS framework that can handle many of these unique functions on OP Chains. It is similar to Ethers.js for OP Stack chains.
Specs (opens in a new tab): For more in-depth technical explanations and examples.